Why does this not compile : List<List<String>> lss = new ArrayList<ArrayList<String>>(); [duplicate]

问题

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Is List<Dog> a subclass of List<Animal>? Why are Java generics not implicitly polymorphic? (17 answers)

Closed 5 years ago.

The below code :

List<List<String>> lss = new ArrayList<ArrayList<String>>();

causes this compile time error :

Type mismatch: cannot convert from ArrayList<ArrayList<String>> to List<List<String>>

to fix I change the code to :

List<ArrayList<String>> lss = new ArrayList<ArrayList<String>>();

Why is this error being thrown ? Is this because the generic type List in List<List<String>> is instantiated and since List is an interface this is not possible ?

回答1:

The problem is that type specifiers in generics do not (unless you tell it to) allow subclasses. You have to match exactly.

Try either:

List<List<String>> lss = new ArrayList<List<String>>();

or:

List<? extends List<String>> lss = new ArrayList<ArrayList<String>>();

回答2:

From http://docs.oracle.com/javase/tutorial/java/generics/inheritance.html

Note: Given two concrete types A and B (for example, Number and Integer), MyClass<A> has no relationship to MyClass<B>, regardless of whether or not A and B are related. The common parent of MyClass<A> and MyClass<B> is Object.

回答3:

Same reason

List<List> myList = new ArrayList<ArrayList>();

wont work.

You can see this question for more details.

来源：https://stackoverflow.com/questions/20702635/why-does-this-not-compile-listliststring-lss-new-arraylistarrayliststr